All disk files require some means of determining the radial position of the READ-WRITE heads over the disks so that the heads can be accurately positioned over any desired track. Typically this is accomplished by placing servo information on one or more of the disk surfaces for reading by magnetic or optical heads. Some disk files, known as dedicated servo files, contain servo information only on a dedicated surface of one disk in the disk stack. In contrast, some modern files, known as sector servo files, store the servo information interspersed with the data on each disk surface. This latter approach represents the direction in which the technology is progressing and is preferred because it can be implemented at low cost and without additional componentry beyond that required to store the data and further because it provides the servo information at the data surface being accessed, thereby eliminating many mechanical and thermal sources of track misregistration. Fixed block architecture (FBA) is a common configuration used to format both dedicated servo disk files and sector servo disk files. In an FBA formatted disk file, each disk track is divided into a number of equal-sized sectors, and each sector is divided into regions containing servo information, identification information (ID), and data.
A typical FBA sector servo sector format according to the prior art is illustrated in FIG. 1A. The sector is divided into three regions: servo and recovery region 10, ID region 11 and data region 12. Generally, servo and recovery region 10 contains overhead associated with sectoring, as well as servo information for sector servo files. Also, this region marks the beginning of the sector. Specifically, servo region 10 contains WRITE-to-READ recovery (W-R) and speed compensation field 13, which is used to switch the data channel from WRITE to READ and accommodate spindle speed variations; address mark (AM) field 14; and servo position field 15 containing a position-error-signal (PES). Data region 12 contains the user data, as well as overhead fields. READ-to-WRITE recovery (R-W) and speed field 19 performs a function like that of W-R and speed field 13. Voltage controlled oscillator sync (VCO) field 20 is used to synchronize the read clock with the read data. Data field 21 contains the user data and associated error checking and correction (ECC) information. ID region 11 is used by the disk data controller (DDC) to identify the physical sector number. It contains R-W and speed field 16, VCO synch field 17 and identification/error handling (ID/EH) field 18, which contains the identification information including the logical sector number.
A typical FBA dedicated servo sector format according to the prior art is illustrated in FIG. 1B. The data disk sector shown is divided into two regions: ID and recovery region 11' and data region 12'. ID and recovery region 11' contains substantially the same information as servo and recovery region 10 and ID region 11 of FIG. 1A, with the exception that servo position field 15 and R-W and speed field 16 are removed. In a dedicated servo scheme, servo position field 15 is not needed because the position information is contained on a separate surface; R-W and speed field 16 is not needed because address mark field 14' can be written whenever the succeeding VCO synch field 17' and ID/EH field 18' are written. Data region 12' contains substantially the same information as data region 12 of FIG. 1A.
ID regions 11 and ID and recovery region 11' of the prior art FBA sector formats perform two important functions. First, they uniquely identify the logical sector number of the sector in which they are located. Second, they indicate whether the sector is good or bad (that is, whether the sector can be successfully written to and read from). Unfortunately, in both the dedicated servo and sector servo scheme, the ID region requires a great deal of disk space to perform its functions. Since the disk uses the information in the ID region to perform a logical-to-physical sector conversion in order to locate the physical position on the disk corresponding to the requested logical sector, the ID region must be large enough to avoid misidentifying sectors. Furthermore, since a phase synchronous clock is typically used to read the ID/EH field the VCO field is required; this field is typically much larger than the ID/EH field. Finally, in the sector servo scheme, due to the region transition an R-W and speed field is required to switch the data channel from READ to WRITE and accommodate spindle speed variations.
In sum, due to overhead and accuracy requirements, the ID region of the prior art FBA sector accounts for a substantial portion of the overall sector length (typically about 4%), a portion that could otherwise be used for data storage. However, the importance of its two functions has hampered efforts to eliminate it from the FBA sector.
One approach that has been used to eliminate the ID region is to encode the ID information into the data field. This eliminates the overhead associated with synching (fields 16 and 17), and allows the ECC to provide error checking for the ID information. However, it does not eliminate or reduce the actual ID information as contained in ID/EH field 18. Also, it adds a minimum of one sector time to the latency during WRITE, since the disk file controller cannot determine the good/bad sector relationship until it has read at least one sector. Further, this scheme is not well suited to compact and low-cost disk files, where the error correction time is typically quite long, since it means a revolution may be lost in the case of a sector where any bit is bad in the entire field containing the combined ID and data. Thus, while the approach of combining the B and data regions reduces the disk space required for information, this approach introduces performance penalties that restrict its usefulness.